Feb. 1, 2011 -- Magnetic resonance imaging (MRI) scans aimed at detecting breast cancer in women at high risk for the disease have fewer false-positive results when conducted over time, according to a new study.

Researchers say women who undergo their first breast cancer MRIs are much more likely to have a false-positive result leading to a referral for additional tests than those who have already had at least one screening MRI scan.

A screening test is described in the study as having a false-positive result when the findings are suspicious for cancer but no cancer is found after further testing or biopsy.

"MRI is an excellent screening tool for breast cancer, but the higher rate of false-positive results keeps some women from undergoing the exam," says one of the study's researchers, Martha B. Mainiero, MD, of Brown University, in a news release. "The goal of our study was to determine if the availability of prior MR images for comparison reduces the rate of false positives associated with the initial MRI breast screening exam."

Reducing False-Positive Results

The research team reviewed reports from 650 consecutive screening MRI breast exams done on women between September 2007 and December 2008 at Rhode Island Hospital in Providence.

The women, who ranged in age from 25 to 81, were referred for an MRI because they were considered to be at high risk for breast cancer.

Of the MRI scans reviewed, 307 were the first the patient had undergone; 343 were annual or repeat screening MRI scans.

In the group receiving their first screening MRI scans, two cancers were identified, for a detection rate of 0.65%. The cancer detection rate in the repeat screening group was nearly twice as high, with cancer being found in four patients, for a rate of 1.17%.

Women undergoing the first or baseline exams were nearly four times more likely to be recommended for a follow-up MRI in six months to monitor findings deemed suspicious compared to patients who had already had one or more MRI exams in the past.

The false-positive rate was 13% in the baseline group and 5.6% in the annual exam group.

Mainiero says in a news release that false positives "are a risk of the breast MRI procedure" but that the rate decreases following the initial round of screening.

Checking MRI Results

The researchers say their study results "confirm that the risk of a patient's having false-positive results" decreases with subsequent screenings. "We presume that this difference is a result of the fact that lesions that have probably benign characteristics will usually be considered benign once there is stability noted when findings are compared with findings of prior examinations," the researchers write.

They say their findings can be used by high-risk patients and their doctors when MRI scans are being considered as an addition to routine mammography screening.

The study is published online in advance of the April print edition of the journal Radiology.